1. Technical field
The present invention relates to a gradient coil incorporated in a magnetic resonance imaging system in order to give magnetic field gradients to an examination space formed in a bore of the system, and in particular, to an active shield type of elliptic cylindrical gradient coil.
2. Related Art
A medical MRI system uses a magnetic resonance phenomenon of nuclear spins within an object to acquire tomographic images or measure NMR spectrums.
The MRI system has a gantry that has an examination space formed, for example, in a substantially cylindrical shape, into which an object to be examined is inserted. The gantry has a static magnet, which forms the examination space, for generating a magnetic static field, a gradient coil for generating magnetic field gradient pulses superposed on the static field, and an RF coil for transmitting and receiving RF pulse signals (including MR signals) to and from the object.
A gradient amplifier, which is coupled with the gradient coil, is driven in response to instructions given from a sequencer to the gradient amplifier, so that the gradient coil creates magnetic field gradient pulses. Currently, it is required that the magnetic field gradient pulses be switched at a switching time of less than 1 msec. Especially, in the case of ultra-fast imaging techniques that have drawn much attention, a switching time of less than 0.3 msec. is required. To realize this, it has been desired that a gradient coil assembly of higher energy efficiency be developed, together with a high-power gradient amplifier.
Conventionally, for MRI systems in which the magnetic static field is directed horizontally, cylindrical gradient coil assemblies have been used. A first measure to increase the energy efficiency of this cylindrical gradient coil assembly is to reduce the coil radius. This is based on the fact that inductance, which is a parameter in determining the switching time of a coil, is proportional to the coil radius to 5th power. However, this technique requires that the coil radius be reduced with an increase in energy efficiency. It was therefore difficult to obtain a radius of the examination space that permits whole-body imaging for adults.
One solution to this problem is proposed for example by U.S. Pat. No. 4,820,988. This publication discloses a gradient coil assembly in which coils are formed into an elliptic cylinder in place of a cylinder. The coil radius is shortened in the vertical direction (normally, in the Y-axis direction) in such a manner that a section perpendicular to the axis through the coils is formed into an elliptic shape. This makes a higher rate of energy efficiency possible and the whole-body examination for adults can be performed. Practically, disclosed coil shapes include a coil shape in which a half cylinder is decentered in the Y-axis direction and placed face to face and a second coil shape in which a cylindrical coil is flattened in a certain direction to the surface so as to form an elliptic cylindrical gradient coil.
An alternative measure is proposed by Japanese Patent Laid-open 5-269100, which discloses an elliptic gradient coil assembly formed into an elliptical cylindrical coil and a detailed design method of such coil assembly.
One significant management item for the gradient coil assembly is to suppress eddy current that occurs in pulsating the assembly to a lower value of current. The eddy current occurs in transition manners when a time-dependent magnetic field penetrates a thermal shielding plate of the static magnet, thereby causing a magnetic eddy field in a region necessary for imaging. This causes irregularities in intensity of an MR image, deeply deteriorating image quality. To prevent this problem, an active (self-) shielded type of gradient coil assembly (ASGC: Actively Shielded Gradient Coil) is used, as shown in for example U.S. Pat. No. 4,733, 189 and U.S. Pat. No. 4,737,716. In such coils, a shield coil is placed outside the gradient coil assembly to suppress or shield a magnetic field leaking out of the gradient coil serving as a main coil. The ASGC has coil assemblies in charge of generating magnetic fields in the individual X-, Y- and Z-channels of an MRI system, and each coil assembly is equipped with a main coil and a shield coil. This shield structure almost prevents, channel by channel, magnetic field gradients from leaking out.
Practical design techniques for an ASGC of which coils are formed into a cylindrical shape are proposed by Mansfield et al., xe2x80x9cJ. Phys. E: Sci. Inst. 19, 540-545 (1986),xe2x80x9d Turner et al., xe2x80x9cJ. Phys. D: Appl. Phys. Vo.19, L147-L151,xe2x80x9d and others.
Thus, an elliptic cylindrical gradient coil should be an actively shielded type of coil. To meet this demand requires a design technique that permits magnetic field gradients to be generated at a higher accuracy. However, such a practical way of designing the elliptic cylindrical gradient coil has yet to be proposed. There are therefore difficulties in designing an active shield type of elliptic cylindrical gradient coil in terms of computing the current distribution of shield coils and computing a desired magnetic performance (such as a maximum gradient strength and linearity in the magnetic field). The active shield type of elliptic cylindrical gradient coil has not been put into practical uses.
The present invention, which has been made with consideration of the above problems that the conventional techniques face, is directed to a detailed manner of designing an active shield type of elliptic cylindrical gradient coil, thereby permitting such a coil to be put into practical uses.
To realize this object, a gradient coil for magnetic resonance imaging (HRI) according to the present invention comprises a first coil forming an elliptic cylinder; and a second coil disposed coaxially to the first coil, wherein a coil wire of the second coil is positioned in winding so that a magnetic field created by the first coil outside the second coil is cancelled.
Preferably, current density in a circumferential direction of each of the first and second coils is expressed by weighted even functions and weighted odd functions, ratios between weights for the second coil and weights for the first coil are determined by values based on the number of waves in a coil-axis direction and a flatness rate of an ellipse, and wound positions of the coil wire of the second coil are determined based on the values.
More preferably, the second coil is located outside the first coil in a radial direction of the first coil. In this structure, the second coil is formed into an elliptic cylinder and an elliptic cylindrical plane formed by the second coil and an elliptic cylindrical plane formed by the first coil possess in common a focus located at almost the same position. By way of example, the second coil is formed into a cylinder.
According to these structures, there can be provided a way to design winding positions of an MRI gradient coil in which the elliptic-cylindrical or cylindrical second coil (shield coil) is disposed outside the elliptic cylindrical first coil (main coil) with the same center axis given. Thus, an active shield type of elliptic cylindrical gradient coil can be provided and put into a practical use, in which a higher rate of energy efficiency is kept, while still enabling whole-body imaging and surely suppressing eddy currents to reduce artifacts.
In addition, the present invention provides a magnetic resonance imaging system in which the above gradient coil is incorporated, a design technique for the gradient coil, and a computer-readable program preferable to the design.